1. Field of the Invention
The present invention relates to a balun (a balun transformer) that performs conversion between unbalanced and balanced signals, and in particular relates to a thin film balun that is formed by a thin film process advantageous for size and thickness reductions.
2. Description of Related Art
A wireless communication device includes various high frequency elements such as an antenna, a filter, an RF switch, a power amplifier, an RF-IC, and a balun. Of these elements, a resonant element such as an antenna or a filter handles (transmits) an unbalanced signal which is based on a ground potential, whereas an RF-IC which generates or processes a high frequency signal handles (transmits) a balanced signal. Accordingly, when electromagnetically connecting these two elements, a balun that functions as an unbalanced-balanced converter is used.
Recently, thin film baluns that are formed by a thin film process advantageous for size and thickness reductions have been increasingly used for mobile communication devices such as mobile phones and mobile terminals, wireless LANs, etc., and in order to meet the need for further size reduction of such devices, further reductions in the sizes and thicknesses of the thin film baluns have also come to be desired. Examples of such thin film baluns which have been proposed include: a chip type balun having a multilayer coil structure (see, for example, reference 1: Japanese Patent Application Laid-Open No. 7-176918); and a balun in which: a magnetic coupling is formed by an unbalanced transmission line and a balanced transmission line which are arranged so as to face each other; one end of the unbalanced transmission line is connected to an unbalanced terminal while the other end is connected to a ground terminal via a capacitor; and an output balanced terminal is connected to the balanced transmission line (see, for example, reference 2: Japanese Patent Application Laid-Open No. 2004-120291).
As a passage characteristic of a thin film balun, a resonant frequency is expressed by formula (1) below.fr=1/{2π(L-C)1/2}  (1),
In formula (1), fr represents a resonant frequency, L (L component) represents an equivalent inductance of a resonant circuit constituted from an unbalanced transmission line and a balanced transmission line, and C (C component) represents an equivalent capacitance of the resonant circuit.
In order to reduce the size and thickness of a thin film balun, the number of turns or the length of lines of coils, etc. which constitute an unbalanced transmission line and a balanced transmission line are inevitably reduced, and thus the inductance L of a resonant circuit will be lowered and the resonant frequency fr (frequency in a pass band) will be increased to a high frequency, as is obvious from formula (1). Generally, a required specification of a passage characteristic in a frequency of a thin film balun used for wireless communication, etc. (an attenuation characteristic in a predetermined frequency) is set based on the configuration, standard, specifications, etc., of a communication device or a system to be equipped with the thin film balun, and the passage characteristic is particularly important in the characteristics of the thin film baluns. Specifically, for example, the peak value of the resonant frequency fr is specified to be in the range between 2400 and 2500 MHz (2.4 GHz band) as the passage characteristic, and the specification is designed so that an attenuation in such a frequency band is sufficiently suppressed to be low. However, if the resonant frequency fr is increased to a high frequency due to the size and thickness reductions of the thin film balun as described above, it will be difficult chip type baluns such as the one disclosed in reference 1 to satisfy such a required specification.
Meanwhile, while the resonant frequency fr can be prevented from being increased to a high frequency by increasing the capacitance C of the resonant circuit as can be seen in formula (1) above, based on the inventors' knowledge, it might be difficult to obtain a desired passage characteristic for a transmission signal only by providing a capacitor in order to simply increase the capacitance as in the thin film balun disclosed in reference 2.